Ink is a liquid or paste that contains pigments or dyes and is used to color a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, or quill. Thicker inks, in paste form, are used extensively in letterpress and lithographic printing. Conventional inks contain solvents, pigments, dyes, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescers, and other materials. These materials control flow and thickness of the ink, and the appearance of the ink when dry.
Ink colorants include pigments and dyes. Pigment inks are used more frequently than dyes because they are more color-fast. Even so, pigments are often more expensive, less consistent in color, and have less of a color range than dyes. Pigments are solid, opaque particles suspended in ink to provide color. Pigment molecules typically link together in crystalline structures that are from 0.1-2 μm in size and usually comprise 5-30 percent of the ink volume. Qualities such as hue, saturation, and lightness vary depending on the source and type of pigment.
Dye-based inks may have better color development than do pigment-based inks, as they can produce more color density per unit of mass. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, making the ink less efficient and potentially allowing the ink to bleed at the edges of an image. To circumvent this problem, dye-based inks are made with solvents that dry rapidly or are used with quick-drying methods of printing, such as blowing hot air on the fresh print.
Chemicals that change color over a range of temperatures are known as thermochromic systems. Thermochromic chemicals can be manufactured to have a color change that is reversible or irreversible. U.S. Pat. No. 5,591,255, entitled “Thermochromic Ink Formulations, Nail Lacquer and Methods of Use”, issued Jan. 7, 1997 to Small et al., discloses methods of producing thermochromic coating formulations without ingredients known to be harmful to thermochromic inks. The use of distilled water as a fountain solution for off-set printing using thermochromic ink is also disclosed.
Thermochromic systems use colorants that are either liquid crystals or leuco dyes. Liquid crystals are used less frequently than leuco dyes because they are very difficult to work with and require highly specialized printing and handling techniques. Thermochromic pigments are a system of interacting parts. Leuco dyes act as colorants, while weak organic acids act as color developers. Solvents or waxes variably interact with the leuco dyes according to the temperature of the system. As is known in the art, thermochromic systems are microencapsulated in a protective coating to protect the contents from undesired effects from the environment. Each microcapsule is self-contained, having all of the components of the entire system that are required for the color change. The components of the system interact with one another differently at different temperatures. Generally, the system is ordered and colored below a temperature corresponding to the full color point. The system becomes increasingly unordered and starts to lose its color at a temperature corresponding to an activation temperature.
Below the activation temperature, the system is usually colored. Above the activation temperature the system is usually clear or lightly colored. The activation temperature corresponds to a range of temperatures at which the transition is taking place between the full color point and the clearing point. Generally, the activation temperature is the temperature at which the human eye can perceive that the system is starting to lose color, or alternatively, starting to gain color. Presently, thermochromic systems are designed to have activation temperatures over a broad range, from about −20° C. to about 80° C. or more. With heating, the system becomes increasingly unordered and continues to lose color until it reaches a level of disorder at a temperature corresponding to a clearing point. At the clearing point, the system lacks any recognizable color.
In this manner, thermochromic pigments change from a specific color to clear upon the application of thermal energy or heat in a thermally-driven cycle exhibiting well-known hysteresis behavior. Thermochromic pigments come in a variety of colors. When applied to a substrate, such as paper, the pigment exhibits the color of the dye at the core of the microcapsules. In one example, when heat is applied generally in the range of 30 to 32° C., the ink changes from the color of the pigment to clear. When the substrate is allowed to return to a temperature under approximately 30° C., the ink returns to the original color of the pigment.
U.S. Pat. No. 5,785,746, entitled “Preparation Method for Shear-Thinning Water-Based Ball-Point Pen Inks Compositions and Ball-Point Pens Employing the Same”, issued Jul. 28, 1998 to Kito et al., discloses reversible thermochromic microcapsular pigment mixed in an ink composition. The microcapsules have concavities to moderate stress resulting from an external force during use in a ball-point pen.
U.S. Pat. No. 5,805,245, entitled “Multilayered Dispersed Thermochromic Liquid Crystal”, issued Sep. 8, 1998 to Davis, discloses a thermochromic substance, applied to inert films in stacked layers with a non-invasive barrier between each thermochromic substance. The thermochromic substance in each layer responds in a different temperature range so that as the temperature changes, each layer repeats a similar sequence of colors. The substrate is a water-based acrylic copolymer formulation coated or permeated with a black pigment. A transparent inert film or non-invasive barrier serves as a protective coating for the thermochromic film and as a support for the next layer of the thermochromic substance.
Specific thermochromic coating formulations are known in the art. See, for example, U.S. Pat. Nos. 4,720,301, 5,219,625 5,558,700, 5,591,255, 5,997,849, 6,139,779, 6,494,950 and 7,494,537, all of which are expressly incorporated herein by reference. These thermochromic coatings are known to use various components in their formulations, and are generally reversible in their color change. Thermochromic; pigments for use in these coatings are commercially available in various colors, with various activation temperatures, clearing points and full color points. Thermochromic coatings may be printed by offset litho, dry offset, letterpress, gravure, flexo and screen processes, amongst others.
Ink pens have previously been developed that have thermochromic inks which can be activated by frictional heat into a colorless state. The colored form of the thermochromic ink cannot be regained without considerable difficulty. For example, reversing the thermochromic transition from colorless to color has previously required difficult and burdensome conditions, such as cooling the thermochromic ink to a temperature of about below the freezing point of water. In addition to being very difficult to regain or reverse the thermochromic transition, previous colored to colorless transitions do not allow for a color to color and/or black to color transitions.